1. Field of the Invention
The present invention relates generally to a BLDC (brushless direct current) motors, and more particularly, to a control circuit for BLDC motors.
2. Description of the Related Art
BLDC motors are nowadays gaining popularity in automotive, consumer, and industrial applications. They have many advantages over traditional brushed DC motors such as higher efficiency, better speed and torque characteristics, low noise generation, wider speed range, less maintenance and longer operating life.
As shown in FIG. 1, a BLDC motor 30 includes three phase windings A, B, and C. To sequentially control the current flowing through the windings A, B, and C, referred to as a known six-step commutation, the BLDC motor 30 will begin to run as the poles of the permanent magnet rotor change angular position. In each step commutation, only two windings are energized.
A driving circuit 25 is coupled to drive the BLDC motors 30 according to the six-step commutation. The driving circuit 25 comprises a plurality of power transistors. A current flowing through two windings is determined by an on-period of the conducted power transistors of the driving circuit 25. For example, while the windings A and B are energized, the variation of the current flowing through the windings A and B which is denoted as ΔIAB can be expressed as the following equation:
                              Δ          ⁢                                          ⁢                      I            AB                          =                                                            V                IN                            -                              E                ⁢                                                                  ⁢                M                ⁢                                                                  ⁢                F                                                    L              AB                                ×          Δ          ⁢                                          ⁢                      T            ON                                              (        1        )            
where VIN is the input voltage; EMF is the back electro-motive force of the BLDC motor 30; ΔTON is the variation of the on-period of the conducted power transistors; and LAB is the equivalent inductance of the windings A and B.
The torque of the BLDC motor 30 is correlated to the current flowing through the conducted power transistors of the driving circuit 25 and the two windings of the BLDC motor 30. Therefore, limiting the current flowing through the conducted power transistors avoids an over-current condition of the power transistors of the driving circuit 25 and also avoids an over-torque condition of the BLDC motor 30.
The power transistors of the driving circuit 25 need to be protected when an excess mechanical load condition of the BLDC motor 30 occurs. The electromotive force varies in direct proportion to the speed of the BLDC motor 30. As the load of the BLDC motor 30 increases, the electro-motive force of the BLDC motor 30 will be decreased in response to the lowered speed of the BLDC motor 30. As equation (1) shows, the current flowing through the two windings of the BLDC motor 30 and the conducted power transistors of the driving circuit 25 will be increased accordingly, which increases the torque of the BLDC motor 30. The increased current might damage the power transistors of the driving circuit 25 and the windings of the BLDC motor 30. Furthermore, the increased torque might damage the mechanical load or cause hurt to the human body.
Power transistors also need to be protected under faulty conditions of the BLDC motor 30, such as locked motor rotor caused from bearing failure or phase winding damages. As the motor rotor is locked, a speed-feedback loop will dramatically increase the on-period of the conducted power transistors of the driving circuit 25 to correct the speed of the BLDC motor 30. However, the dramatically increased current might cause permanent damages to the conducted power transistors of the driving circuit 25 without protection circuits.
Therefore, there is a need to provide a control circuit to properly limit the current flowing through the conducted power transistors of the driving circuit 25 for preventing the damages to the conducted power transistors and the mechanical load or avoiding the human body being hurt by the excess torque of the BLDC motor 30.